This invention is concerned generally with a method and apparatus for studying the surface properties of a sample or testpiece, for example the refractive index or the thickness of a layer or film on the surface, using electromagnetic radiation which is reflected at the surface to be studied, with reflected, linearly polarised radiation being extinguished by an analyzer.
Ellipsometer instruments have been proposed for determining the optical properties of sample or testpiece surfaces, which operate on the principle that parallel monochromatic light falls on the surface through a polarizer which provides for linear polarization of the light. The light then passes through a compensator which produces a phase difference between the polarization components of the light, thus producing elliptical polarization of the light. After reflection of the light at the surface of the testpiece, the light is analyzed and calculation makes it possible to achieve information about the quality and nature of the surface at which the light was reflected.
If the surface of the sample or testpiece serves as the substrate for a dielectric film, the reflection properties of that surface are influenced. From this, it is possible to calculate the thickness and the refractive index of the film, if the optical constants of the substrate are known. The degree of accuracy in determining thickness and refractive index vary according to the nature of the substrate, the angle of incidence and the thickness of the film. If the substrate comprises silicon, and if the angle of incidence of the radiation on the sample or testpiece is 70.degree., the thickness of the film may be determined with an accuracy of measurement of .+-.0.05 nm.
However, when measuring the thickness of thin films, it is necessary for the thickness to be known to approximately half the wavelength of the radiation, as the setting or adjustment angles of the polarizer and the analyzer are periodically repeated; if there is no approximate reference point or criterion in regard to measuring the thickness of the film, before the measuring operation is performed, difficulties can occur as the correct values must be selected from a number of differing values. In this respect, besides the data determined with the ellipsometer, it is also necessary to have additional information which makes it possible to decide which is the correct thickness value within a multiplet of a plurality of measurement values.
Therefore, an ellipsometer of such a design is only capable of limited use. Although the ellipsometer is relatively simple in design, the measuring operation turns out to be relatively complicated, particularly because the measurement values must be determined in dependence on polarizer and analyzer settings, and because, after the measuring operation, extensive calculations are still required in order to arrive at the actual measurement values.
Also known are automatic ellipsometers which have servo and computer equipment which carry out the manual calculation work. However, ellipsometers of this kind are expensive and complicated pieces of equipment.